WEST LAFAYETTE, Ind. - The
crystallized form of a molecular machine that can
cut and paste genetic material is revealing possible
new paths for treating diseases such as some forms
of cancer and opportunistic infections that plague
Purdue University researchers
froze one of these molecular machines, which are chemical
complexes known as a Group I intron, at mid-point
in its work cycle. When frozen, crystallized introns
reveal their structure and the sites at which they
bind with various molecules to cause biochemical reactions.
Scientists can use this knowledge to manipulate the
intron to splice out malfunctioning genes, said Barbara
Golden, associate professor of biochemistry. Normal
genes then can take over without actually changing
the genetic code.
The results of the Purdue study
are published in the January issue of the journal
Nature Structural and Molecular Biology.
"In terms of human health,
Group I introns are interesting because they cause
their own removal and also splice the ends of the
surrounding RNA together, forming a functional gene,"
Golden said. "We can design introns and re-engineer
them so they will do this to RNA in which we're interested."
Once thought of as genetic
junk, introns are bits of DNA that can activate their
own removal from RNA, which translates DNA's directions
for gene behavior. Introns then splice the RNA back
Scientists are just learning whether many DNA sequences
previously believed to have no function actually may
play specialized roles in cell behavior.
While humans have introns,
they don't have Group I introns. Many pathogens that
cause human diseases, however, do have Group I introns,
including the HIV opportunistic infections pneumocystis,
a form of pneumonia, and thrush, an infection of tissues
in the oral cavity. This makes introns a potential
target for therapeutics against these diseases by
using a strategy called targeted trans-splicing in
which introns are manipulated to cut out malfunctioning
Introns' unique capability
of cutting and pasting apparently has been conserved
since life evolved.
"It's thought that RNA,
or a molecule related to RNA, possibly were the first
biomolecules, because they are capable of both performing
work and carrying around their own genetic code,"
She and her research team used
an intron from a bacteriophage, a molecule that attacks
bacteria, to obtain an intron crystal structure trapped
in the middle of the cutting and pasting cycle. As
introns proceed through their work cycle, they change
shape by folding and bending. By crystallizing the
complex at various stages, the scientists can determine
and study its three-dimensional structure and learn
how it is able to carry out its biochemical work.
The Group I intron at its work
cycle's mid-point, which Golden crystallized, is unreactive
but reveals many of the interactions between the RNA
and the molecules that it activates, she said.
"Knowing the structure
can help us engineer molecules to behave better,"
Golden said. "It's very hard to find targets
in cells because cells are organized in ways we still
don't fully understand. This crystal structure shows
us where the best targets are for modifying genetic
The crystal structure of this
Group I intron also will allow scientists to form
models of hundreds of other introns in the same family
and provide possibilities for new treatments for a
wide variety of diseases, she said.
Other scientists now will use
the information gleaned from this study in an attempt
to develop new drugs, Golden said.
Introns were unknown until
the late 1970s, and scientists are still investigating
their function. Crystallization of the complex is
one tool to determine their purpose.
Two intron structures in different
stages of the cycle have been crystallized previously,
and the targeted trans-splicing technique has been
used to repair hemoglobin infected with sickle cell
anemia. The new structure provides scientists with
tools to expand on ways to harness this molecular
machine, Golden said.
The other researchers on this
study were Hajeong Kim, graduate student, and Elaine
Chase, research associate, both of the Purdue Department
of Biochemistry. Golden also is a scientist in the
Purdue Cancer Center, a National Cancer Institute
designated research facility.
Grants from NASA, the Pew Scholars
Program in Biomedical Sciences and the Purdue Cancer
Center provided funding for this research.
Writer: Susan A. Steeves, (765)
Source: Barbara Golden, (765)
Related Web sites:
Purdue Department of Biochemistry:
Purdue Cancer Center: http://www.cancer.purdue.edu/
National Cancer Institute:
Nature Structural and Molecular